The present invention relates to a method for obtaining a sintered body of carbonitride alloy with titanium as the main component and which does not have a binder phase layer on the surface after sintering. This has been achieved by processing the material in a specific way to obtain poor wetting of the binder phase on the surface, essentially without depth effect.
Titanium-based carbonitride alloys, so-called cermets, are well established as insert material in the metal cutting industry and are especially used for finishing. They consist of carbonitride hard constituents embedded in a metallic binder phase.
In addition to titanium, group VIa elements, normally both molybdenum and tungsten and sometimes chromium, are added to facilitate wetting between binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, e.g., Zr, Hf, V, Nb and Ta, are also added in all commercial alloys available today, usually as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is usually &lt;2 .mu.m. The binder phase is normally a solid solution of mainly both cobalt and nickel. The amount of binder phase is generally 3-25 wt %. Furthermore, other elements are sometimes used, e.g., aluminum, which are said to harden the binder phase and/or improve the wetting between hard constituents and binder phase. Of course commercially available raw material powders also contain inevitable impurities. The most important impurity is oxygen, due to its high affinity to titanium. A normal impurity level for oxygen has historically been &lt;0.3 wt %. Recently, due to improved production methods for titanium-based raw materials, this level has been possible to decrease to &lt;0.2 wt %, especially for grades with low nitrogen content. Very high oxygen levels are generally avoided since this may cause formation of CO gas after pore closure, which in turn leads to excessive porosity.
Common for all cermet inserts is that they are produced by the powder metallurgical methods of milling powders of the hard constituents and binder phase, pressing to form bodies of desired shape and finally, liquid phase sintering the pressed bodies. During sintering, the bodies are heated above the eutectic temperature for the composition to form a liquid binder phase. Provided that good wetting is obtained between the liquid and the solid hard phase grains, strong capillary forces are obtained. The action of these forces is to shrink the porous body essentially isotropically, eliminating porosity. The linear shrinkage is typically 15-30%.
After such sintering, the cermet inserts are covered with a thin, continuous binder phase layer on the surface, typically 1-2 .mu.m thick. This is a natural consequence of the good wetting. The presence of binder phase on the surface gives the inserts a nice metallic luster but is not desirable for at least three reasons:
1. For mass balance reasons, a shallow binder phase depletion is obtained just below the surface, adversely influencing the toughness of the material. Both the magnitude and the width of this depletion are difficult to control. PA1 2. During the initial stages of cutting, before the binder phase layer has worn off, there is a significant risk that the chip from the work piece will be welded to the binder phase layer close to the cutting edge. Subsequently, when the chip is torn away, the cutting edge is damaged. PA1 3. If the insert is to be coated with a thin wear resistant coating, the binder phase on the surface will decrease adhesion and quality of the coating.
Methods available today to remove the binder phase surface layer include chemical etching, grinding, blasting or brushing. All these methods represent expensive extra production steps and also have other disadvantages, e.g., preferential material removal, difficult process control and risk for surface corrosion.